Abstract
Unmineable coal seams constitute important domestic sources of natural gas in several countries. Although limited in size, coal seams have the advantage of being naturally fractured, which facilitates drainage upon depletion. Reservoir depletion changes the state of stresses in the reservoir. The change of stress is suggested by order-of-magnitude increases of permeability upon depletion and observation of significant fines production in some mature wells, this latter phenomenon known as "coal failure". Desorption-induced shrinkage of coal appears to cause changes of stress in the formation in addition to those of porepressure reduction during depletion. We summarize laboratory data replicating the stress path of coal seams far from the wellbore considering no change of horizontal strain and constant total vertical stress. Experimental results show that desorption-induced shrinkage promotes significant lateral stress relaxation sometimes leading to shear failure. Desorption amplifies the effect of pore pressure reduction and makes shear fracture reactivation more likely to happen than in non-sorbing rocks. This study shows the importance of considering depletion in coal seams as a chemo-geomechanical coupled process and the importance of rock failure in bulk reservoir permeability.
1. INTRODUCTION
It has been long known that depletion of water- and hydrocarbon-bearing reservoirs can change the state of stresses in the subsurface. Changes in stress can affect both the reservoir and surrounding formations [1-2]. Pore pressure reduction associated with depletion commonly causes an increase of effective stress in the reservoir rock such that the reservoir rock suffers a process of compaction. This change of stresses in the reservoir is usually anisotropic. Effective vertical stress is expected to increase more rapidly than horizontal vertical stress in horizontally extensive reservoirs. Such anisotropic changes in stress can cause shear failure in the reservoir rock. These changes are commonly measured as a reduction of total horizontal stress as documented in fields in the North Sea and the Gulf of Mexico [3-5]. Significant changes of effective stresses upon depletion are also expected in the vicinity of the wellbore and perforations. Localized failure in these areas coupled with high flow rates leads to production of fines, a phenomenon known as sand production [6-7].
Coal seams are naturally fractured gas reservoirs [8-9]. They are usually laterally extensive with modest net thickness. For example, the net thickness of the Fruitland Formation in San Juan Basin varies from less than 1 m to ~20m [10].
